Liver is a unique tissue which is able to regenerate in response to partial hepatectomy (PH) and after injury. My laboratory investigates the role of transcription factor CCAAT/Enhancer Binding Protein alpha, C/EBP1, in the control of liver proliferation. C/EBP1 is a very strong inhibitor of proliferation. The ability of C/EBP1 to inhibit cell proliferation mainly depends on the direct interactions of C/EBP1 with cell cycle proteins and these interactions are controlled by certain signal transduction pathways such as cyclin D3-cdk4/6 and PI3K-Akt-PP2A. Cyclin D3-cdk4 phosphorylates C/EBP1 at Ser193 and increases its interactions with cdk2 and Brm resulting in the inhibition of cell proliferation. On the contrary, the activation of Akt-PI3K-PP2A pathway leads to the de-phosphorylation of C/EBP1 and to the loss of a negative control of proliferation. We have generated C/EBP1 knock-in mice in which the endogenous gene is replaced with constitutively active C/EBP1-S193D mutant. The C/EBP1-S193D is no longer under PI3K-Akt-PP2A control and is resistant to the neutralization of the inhibitory activity. We found that proliferation of the liver is inhibited by the constitutively active C/EBP1 and that the S193D mutant displays additional functions which regulate biology of the liver. The nuclei of the S193D knockin mice have increased size and changed chromatin structure which are caused by the elevation of C/EBP1-HDAC1 complexes. These complexes occupy the promoters of cyclin D1 and D2. In addition, we identified cdc2 as a kinase which interacts with C/EBP1 at late stages after PH and restores growth inhibitory activity of C/EBP1 via phosphorylation at S193. The main hypotheses of the application are that the phosphorylation of C/EBP1 at Ser193 is critical for 1) transition of the liver from proliferation to quiescence during postnatal liver differentiation; 2) protection of livers from tumor development; 3) formation of C/EBP1-HDAC1 complexes and following repression of cyclin D1; and 4) termination of proliferation when the liver finishes regeneration after PH. Specific Aim 1 examines the role of phosphorylation of C/EBP1 at S193 in the transition of liver from proliferation to quiescence during postnatal development and the role of this phosphorylation in protection of livers from tumor development. Specific Aim 2 examines the role of C/EBP1-HDAC1 complexes in the epigenetic silencing cell cycle proteins and in inhibition of liver proliferation. Specific Aim 3 will examine the hypothesis that cdc2-mediated phosphorylation of C/EBP1 is required for the exit of hepatocytes from cell cycle when liver finishes regeneration. The understanding of the mechanisms of the inhibition of liver proliferation is necessary for generation of therapeutic approaches to prevent development of tumor in the liver.